The present invention relates, in general, to non-destructive ultrasonic testing and, in particular, to a new and useful apparatus and method for detecting the size, depth, orientation and location of flaws in material by sensing the change in phase of an ultrasonic echo pulse.
Conventional ultrasonic testing provides time, amplitude and spatial information that is combined through mechanical and electrical apparatus to form A, B and C scans for flaw detection and sizing. The most commonly used technique is time-amplitude or A scan ultrasonic testing. All of these techniques make use of either a continuous wave or a pulse excitation and time reference signal. The primary limitation of these techniques are that they only allow discrimination via signal ampitudes, time separation or spatial separation. These parameters are insufficient to independently define the size, depth, orientation and location of a flaw.
In coarse-grained materials, ultrasonic energy which is backscattered at grain boundaries, provides a signal which is continuous during the time that pulse echoes are being received. These backscattered signals may totally obscure the echoes. Evidence shows that if a defect is present to reflect the ultrasonic energy, the phase of the composite echo may be used to indicate the presence of the defect even though a distinctive pulse may not appear above the noise. The noise is commonly referred to as "grass". Unfortunately, as the transducer used to generate and receive the ultrasound is scanned over the surface, the amplitude will reach a single or at most several peaks before decaying. In contrast, phase would continuously change in one direction with transducer motion, stop when the transducer is at the point of closest approach, and reverse. A single phase measurement is not sufficient. What is needed is a device to sense the change in phase with transducer position. Also, it must indicate the difference between random phase variations obtained with only grain boundary reflections, and a distinctive pattern obtained with a reflector such as a flow.
U.S. Pat. No. 4,253,337 to Vasile discloses an ultrasonic testing method which utilizes phase measurements and/or phase shifting of ultrasonic materials. Vasile discloses a method of evaluating a defect or discontinuity in an object. As such the reference utilizes the transmission of an ultrasonic wave into the object and the detection of the wave after it has propagated through the discontinuity. The depth of the discontinuity is also considered with respect to the change in phase and amplitude of the detected wave which are compared to values of a wave propagating in the absence of a discontinuity. Vasile does not however detect the change in phase of the echo pulse with transducer position and does not use two phase detectors utilizing in-phase and quadrature reference signals. In addition, a rotating vector display is not utilized which is used in the present invention as will be explained fully hereinunder.
U.S. Pat. No. 4,003,244 to O'Brien et al discloses an ultrasonic pulse echo thickness apparatus wherein the search signal and the echo signal are amplified so as to be in equal magnitude and are also phase shifted so as to be in phase with each other. In this manner, overall system accuracy is increased. Thus, this reference deals with the phase of the signal, however, it does not teach the sensing of a change in phase of an echo pulse with transducer position. O'Brien also does not utilize other features of the present invention.